Moving mirrored UVs to other quadrant for baking
In the UV editor check the Coordinates Normalized property (Properties Panel > Display).

Instead of moving them manually in grab mode, you can enter values in grab, rotate or scale mode. They will appear in the menu bar of the active area (image editor). Minus - and Backspace work, while entering a value in a transformation mode.

To move some faces 100% out of the uv space, press the following: G > -1 > Enter.

With normalized coordinates many similiar transformations can be made, for example flipping or flopping vertices. Just set the cursor coordinates to (0.5, 0.5) and the Rotation/Scaling Pivot to 2D Cursor. Then scale with a value of -1 along x or y. (S > X > -1 > Enter)